In a wireless network, a wireless device may communicate with one or more radio access nodes to send and/or receive information, such as voice traffic, data traffic, control signals, and so on. One method wireless network operators use to cope with the increasing number of mobile broadband data subscribers and bandwidth-intensive services competing for limited radio resources is to add small cells within their macro networks to spread traffic loads, maintain performance, and use spectrum efficiently.
Small cells comprise low-powered radio access nodes that can operate in licensed or unlicensed spectrum that have a range of approximately 10 meters to 1 or 2 kilometers. This range is “small” compared to a macrocell, which may have a range of a few tens of kilometers. Small cells may include, for example, femtocells, picocells, and microcells. Small-cell networks may also include distributed radio technology consisting of centralized baseband units and remote radio heads. Sometimes beamforming further enhances small cell coverage. Small cells are available for a range of radio interfaces including Global System for Mobile Communications (GSM), CDMA2000, TD-SCDMA, W-CDMA, Long Term Evolution (LTE) and WiMax. In 3GPP terminology, a Home Node B (HNB) is a 3G femtocell. A Home eNode B (HeNB) is an LTE femtocell.
Efficient operation in densely deployed small cells relies on low interference between cells. A particular mechanism to reduce interference between cells is known as small cell on/off. Small cell on/off provides energy-efficient load balancing by turning off the low-power nodes when there is no ongoing demand for data transmission. More evolved Node Bs (eNBs) increases radio interference and network power consumption. Making nodes dormant can match available capacity to network traffic loading. Small cell on/off may also provide energy savings.
The particular on/off scheme in use, depending on its time scale, can have an impact on user equipment (UE) measurements. This is because the particular reference signals to be measured must be available during the measurement time period. To facilitate measurement on a cell, even when the cell is off, a schedule for transmission of discovery signals must be coordinated. These discovery signals will be transmitted with a lower periodicity than reference signals that are usually transmitted when the cell is on.
For making measurements on currently used component carriers, measurement cycle lengths are configured for a UE. The measurement cycle configuration includes requirements on the measurements that must be made within the measurement cycle. To perform measurement on cells operating in carrier frequencies that are different from the ones that the receiver in the UE is currently tuned to, the network node schedules measurement gaps so that the UE can tune one or more of its receivers to other frequencies during the measurement gaps to make the measurements.
When small cell on/off is used on the cells in the current component carriers or used in other frequencies, the occurrence of discovery signals on these cells should occur often enough in the measurement cycle and should be aligned with the measurement gaps so that a signal is available for the UE to measure. A problem that arises with the low periodicity of discovery signals and the low periodicity of measurement gaps is that at a particular time all (or at least a large fraction) of UEs may be unavailable to send/receive data on the serving cell because they are scheduled to make measurements on the other cells that are transmitting discovery signals with low periodicities.
The measurement procedure adopted by a UE may depend on the type of information that the network provides to the UE. As a specific example, the measurement procedure may depend upon the particular reference signal that the UE is to measure. Under normal cell operation, the network node transmits the reference signals used for measurements periodically and also frequently. In particular examples, the network node may transmit reference signals in every subframe. In such an example, a UE measurement configuration for a deactivated secondary cell (SCell) can be determined using the SCell measurement cycle (i.e., measCycleSCell), which is configured by higher layers. Also, measurement gaps are provided for performing inter-frequency measurements. A radio network node refrains from scheduling a UE on the serving cell during these measurement gaps.
When small cell on/off is operated on serving component carriers or on cells operating in frequencies other than the current primary component carrier (PCC) and secondary component carriers (SCCs), coordination is necessary to ensure that the occurrence of discovery signals on these cells are aligned with the measurement cycle periods and measurement gaps so that a signal is available for the UE to measure. One problem that arises with the low periodicity of discovery signals and measurement gaps is that all (or a large fraction of) UEs may simultaneously not be available on the serving cell for sending data, which is undesirable. Another problem is when multiple frequencies use the same aligned subframes for discovery signal transmission, which significantly increases the time for a UE to make inter-frequency measurements because the UE has to tune its receiver sequentially to each frequency and measure the cells on the frequency before proceeding to the next one. Therefore, efficient solutions are needed that do not negatively impact measurement quality while also making enough UEs available for scheduling of data in the serving cells.